For Vodafone Group Plc , Bharti Airtel Ltd. and other phone companies with more than $90 billion invested in Africa, making money from each user in the world’s fastest-growing market is turning into the biggest challenge.

Facebook Inc. founder Mark Zuckerberg is thinking big about how to increase Internet access in the developing world. That’s an ambitious and laudable goal. Incidentally, it could also be very profitable for Zuckerberg’s company.